As you probably are aware, a dihedral wing configuration provides roll stability. Roll the plane, and it will naturally roll back to level. A center of mass well below the center of lift provides the same effect. Put a lot of weight down below the center of lift, and you'll get the roll-back effect again.

While that's nifty, you can get too much of it. Too much stability makes the plane very hard to turn or otherwise control. You can also get a sort of aerodynamic pendulum-like effect called a "Dutch roll" as the "roll back" action over-corrects and causes a swing back the other way, and back and forth with a combination yaw and roll that is surprisingly effective at inducing air-sickness. Putting an anhedrial angle on the wing counteracts the "too-much-stability" problem created by the weight distribution, making the plane easier to control again and reducing unwanted oscillations.

Why not just make the wings lower? That may solve the stability problem, but it would create other problems as well. Just how low would the wings have to be? Would that design affect safety for ground operations or unpaved runways? And what about cargo -- how well does your new design perform in empty versus loaded configurations? How tall would the landing gear have to be to make such a design safe... and how does that affect loading and unloading?

Some aircraft, even the comically large Airbus A300-600 Super Transport "Beluga", opt for the low-wing solution, which invariably leads to a dihedral.

But in certain cases, the high-wing plus pronounced anhedrial design has won out for some heavy cargo hauling designs, and is particularly popular with military where they may have to operate out of dirt runways and need to get the wings up away from the ground, and where short landing gear is desirable.

$\begingroup$The best explanation so far. Just keeping the gear short is mainly important to make loading easier; An-224 is too heavy for unpaved runway anyway.$\endgroup$
– Jan HudecJun 12 '14 at 20:10

$\begingroup$Sorry to disagree, Jan, but Radu has my vote this time. Dutch roll damping is the main reason for anhedral with high-mounted wings. Once you start yawing, the low fuselage helps to roll the aircraft, so roll maneuverability is not the issue.$\endgroup$
– Peter KämpfJun 12 '14 at 20:28

$\begingroup$@PeterK&#228;mpf dutch roll is yet another side-effect of excessive stability. You can solve it with several options, but anhedral wing mounting is the one that keeps the wings up high. Other planes, like the airbus a380 have low-mounted wings with a dihedral, and still no dutch roll.$\endgroup$
– tylerlJun 13 '14 at 0:04

$\begingroup$@tylerl: Sure, that is due to the fuselage influence on wing clbeta. The Antonov has high wings so tanks can drive all the way through, and the 380 has low wings so it looks familiar to passengers, although it would have benefitted from a mid wing configuration. The fact stays, the 225 anhedral is mainly due to dutch roll damping.$\endgroup$
– Peter KämpfJun 13 '14 at 16:41

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$\begingroup$The Beluga isn't a good example either. No explicit dihedral/anhedral design choice was made for the Beluga, as it is a variation of the original A300. To ease certification , as much as possible was left untouched, thus giving if the same wing configuration as the original A300.$\endgroup$
– ROIMaisonMar 13 '15 at 16:14

If you look for pictures of the Mriya in flight the wings are more-or-less level. On the ground, fully fueled, the wings with 3 engines each are heavy and will bend down a very noticeable amount. The B-52 has a similar issue, to the point where it has outrigger wheels near the end of the wings to keep them from scraping the pavement.

It is not unusual for a dedicated cargo plane design to have a high mounted wing design. It allows for the fuselage of the plane to sit lower on the tarmac while maintaining the ground clearance for the engines and wingtips.

It also makes it easier to allow the cargo door to be opened in flight without compromising the structural integrity because the main strength of the fuselage comes from the spine in the top where the wings and tail attach to.

Looking at the list of military transport aircraft, most use the same wing design. The only exceptions are those based on a civilian aircraft.

$\begingroup$You've overlooked the "anhedral" side of the question. Is it really so or not as argued @MikeFoxtrot?$\endgroup$
– menjarazJun 12 '14 at 9:50

$\begingroup$@menjaraz on ground, especially for large wings, the angle is always different from the flight one, but, afaik, the angle is measured and defined on ground.$\endgroup$
– Federico♦Jun 12 '14 at 10:42

The wing is so long that it will bend significantly when lifting forces are applied. Therefore an initial anhedral angle is present to correct for some of this bending, as without it the resulting dihedral in high lift situations would be excessive and would cause a lack of roll control.

Part of the reason for using an anhedral wing on cargo aircraft is that at speed, the wings lift and are straight out. If you look at the B-52, the wings are so long that even though they need the outriggers when on the ground, the wings flare up while in flight.

$\begingroup$That may very well be true of the B-52 at heavy loadings, but not of many of the Antonev aircraft with their very pronounced anhedral. See in-flight photos.$\endgroup$
– quiet flyerOct 27 '18 at 15:20

$\begingroup$Vertical position of the centre of mass does not actually have any effect on stability. Adding cargo on the roof does have effect on the yaw-roll coupling though do to the drag of that cargo though.$\endgroup$
– Jan HudecOct 27 '18 at 20:51

$\begingroup$Is it easier to balance an object by holding it from underneath, or by holding it from the top? Interesting to note the mythical pendulum is still alive in returning a rolled aircraft to upright. But lowering CG may also have an aerodynamic effect in a side slip by presenting more area above CG. Sort of a trade off, easier to start roll, more of a tendency to return to upright. Adding more area beneath CG lessens aerodynamic tendency in side slip to roll, as does anhedralling. But the mighty Antonov does have quite a bit of sweep too!$\endgroup$
– Robert DiGiovanniOct 28 '18 at 6:33

$\begingroup$One might try to render the shuttle weightless at cruise speed by setting its pitch just right! But low and slow, I would imagine they would keep it super coordinated. While back someone posted CG specs for 747 shuttle carrier. They seemed very strict for pitch too.$\endgroup$
– Robert DiGiovanniOct 28 '18 at 11:34

It is interesting that the some of the lightest aircraft in the world-- flex-wing hang gliders and powered "trikes" -- have a similar anhedral configuration to the massive An-225. The reason in the case of the hang glider is to maintain good roll response even in the presence of sideslip due to adverse yaw. I.e. to keep the wing's overall "effective dihedral"-- the roll torque generated by sideslip-- close to zero in the part of the flight envelope where handling is most critical, despite the fact that the swept-wing or delta-wing geometry contributes a dihedral-like roll torque in the presence of sideslip.

In a modern hang glider, the pilot is freely hanging by a strap connected near the CG, so when he is not exerting a force with his muscles his weight effectively acts at the CG, so there is neither the "pendulum effect" (CG well below center of drag of wing) nor the aerodynamic interference between the fuselage and wing (see https://www.av8n.com/how/htm/roll.html#sec-other-slip-roll ) which contribute to shift the "effective dihedral" in the positive direction in a high-winged aircraft like the An 225.

With a modern cargo jet adverse yaw may be well-controlled by other means, but too much "effective dihedral" (slip-roll coupling) due to sweep, low CG, and high wing placement still could create unpleasant handling in strong crosswinds, and could contribute to Dutch Roll oscillations. There could also be adverse consequences if the pilot failed to prevent the aircraft from yawing sideways to the airflow upon loss of power from one or more engines. (See related comment within this answer to another question -- https://aviation.stackexchange.com/a/56481/34686 )

When thinking about roll stability, it is important to remember that slip-roll coupling -- i.e. positive "effective dihedral" -- is a key part of roll stability and is the reason that a high-wing plane will tend to be more stable (or less unstable), in terms of roll stability or spiral stability, than a low-wing plane designed with the exact same physical wing shape including dihedral angle. The high-wing will also be easier to maneuver through banked turns using the rudder alone, via slip-roll coupling. (A fun exercise to try -- ask yourself "could I land this plane if the control yoke fell off?) But the high-wing plane will be less pleasant to handle in strong gusty crosswinds, and will suffer more of a roll rate penalty if the pilot doesn't use the rudder properly and allows the aircraft to adverse-yaw and sideslip while entering a turn.